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110 MORPHOLOGY AND EVOLUTION
is short and broad, the angle is inflected to a greater or less extent; and the coronoid process exhibits a variable degree of recurving of the point even in the same species. The body of the bone is thickened round the incisor, canine and first premolar teeth, and its vertical height is greatest in these regions.
Keith (99) points out that the skull of the Siamang can be distinguished from those of all other Gibbons, but it is not possible to differentiate those of the various species of the genus Hylobates from one another. Perhaps the examination of a hundred or more skulls of each species might enable us to discover specific characters.
The most characteristic feature in the vast majority of Orang skulls is the hollowing out of the middle of the face, a condition to which the term u simognathism" is applied. The degree of simognathism varies with age, sex and the locality from which the animal was obtained; so a large series of skulls exhibits marked differences in profile. Thus the facial outline may be nearly, but not quite, straight; or the skull appears constricted in the middle, thus showing up prominently the size of the cranium and jaws (fig. 221).
The skull resembles that of any Gibbon in a number of points, but differs from it in many respects, first and foremost of which is size. The absolute bulk is greater, the jaws are more massive, and the cranial capacity is
* The skull is described in whole or in part in papers 15, 20, 92,
109, 122, 235, 245, 246, 254, 26l, 265, 267, 276, 280, 343' , 380, 451,